Christians around the world are celebrating the central event of their faith - Easter - the holiday that marks what Christians believe is the resurrection of Jesus three days after his crucifixion.

Pope Francis prayed for an end to the persecution of Christians in his Easter Sunday address, commemorating the students massacred by Islamist militants at Garissa University College in Kenya.

Francis, after saying Mass for thousands of people in a rainy St. Peter's Square, delivered a mostly sombre "Urbi et Orbi" (to the city and the world) message.

Attacks on Christians in Africa and the Middle East have been the grim backdrop of all Holy Week ceremonies leading up to Easter.

"We ask Jesus, the victor over death, to lighten the sufferings of our many brothers and sisters who are persecuted for his name, and of all those who suffer injustice as a result of ongoing conflicts and violence - and there are many," he said.

The pope spoke as churches in Kenya, where al-Shabab gunmen massacred nearly 150 people, singling out Christians for point-blank executions, turned to armed guards to protect their congregations on the most important day of the Christian liturgical year.

"May constant prayer rise up from all people of goodwill for those who lost their lives - I think in particular of the young people who were killed last Thursday at Garissa University College in Kenya - for all who have been kidnapped, and for those forced to abandon their homes and their dear ones," Francis said.

Rainy Easter Mass

The 78-year-old Argentine pope, celebrating the third Easter of his pontificate, spoke from the central balcony of St. Peter's Basilica after saying a Mass below for tens of thousands of people wearing plastic ponchos and holding umbrellas against the driving rain.

Calling for peace in Libya, where last February, Islamic State militants beheaded 21 Egyptian Coptic Christians, the pope called for an end to "the present absurd bloodshed and all barbarous acts of violence."

He prayed for peace in Syria, Iraq and Yemen, Sudan, South Sudan, the Democratic Republic of Congo and Nigeria, where Boko Haram Islamist militants have also targeted Christian churches.

"We ask for peace and freedom for the many men and women subject to old and new forms of enslavement on the part of criminal individuals and groups," he said.

"Peace and liberty for the victims of drug dealers, who are often allied with the powers who ought to defend peace and harmony in the human family. And we ask peace for this world subjected to arms dealers, who make their money from the blood of men and women" he said.

Just about the only positive part in the pope's address was a reference to the deal reached in Switzerland last week between Iran and the international community on a framework for a nuclear accord.

"In hope we entrust to the merciful Lord the framework recently agreed to in Lausanne, that it may be a definitive step toward a more secure and fraternal world," he said.

In the United States, President Barack Obama and his family attended Easter services at the Alfred Street Baptist Church in Alexandria, Virginia, which claimed its history goes back to when Thomas Jefferson was in the White House.

The church's website said it was founded in 1803 and a slave was baptized that year as its first black member.

Easter celebrations in Jerusalem

Meanwhile, in Jerusalem, thousands of Christians gathered at the site many Christians believe Jesus was crucified and buried.

Easter celebrations began in Jerusalem with a sunrise service at the Garden Tomb. Pilgrims sang hymns of the resurrection in the scenic garden, which resembles the place where Jesus was buried according to biblical accounts. Facing an empty tomb carved into a rock, they proclaimed that Jesus is risen.

Latin Patriarch of Jerusalem Fouad Twal, center, takes part in the Easter procession in the Church of the Holy Sepulchre in Jerusalem's Old City, April 5, 2015.

It was a more solemn service at the Church of the Holy Sepulcher in the Old City.

Priests and monks in festive robes chanted the Easter liturgy in front of the ancient stone tomb revered by Christians as the place where Jesus rose from the dead.

Pilgrims came from all over the world. “I have had an amazing spiritual experience here in Jerusalem as the place that Christ chose to come,” said Rick Keller is from Atlanta, Georgia in the United States. “There is a sense of connection here that is much greater than maybe a lot of places in the world. I think there is a spirit about this city.”

Shirley Relosa, who came from Manilla in the Philippines, said, “It is a good feeling being here. It is divine and magical. The atmosphere is good, very peaceful. Everybody is smiling, everybody is inviting.”

Jerusalem is packed with both Christian and Jewish pilgrims as Easter coincides this year with the weeklong biblical holiday of Passover.

Christians believe Jesus Christ is the Son of God, and was resurrected from the dead in order to grant eternal life and forgiveness of worldly sins to his followers.

In the United States, there are a number of Easter customs that Christians participate in, including attending sunrise church services, and dyeing and decorating eggs.

Eastern Orthodox Christians celebrate Easter next week.

Robert Berger contributed to this report from Jerusalem. Some material for this report came from Reuters and AP.